The rate, published Thursday by Freddie Mac, is the lowest since the mortgage finance company began tracking the data in 1971. The previous record of 4.78 percent was set during the week ending April 30 and matched last week.
The Federal Reserve is pumping $1.25 trillion into mortgage-backed securities to try to bring down mortgage rates, but that money is set to run out next spring. The goal of the program is to make home buying more affordable and prop up the housing market.
Despite the government support, qualifying for a loan is still tough. Lenders have tightened their standards dramatically, so the best rates are available to those with solid credit and a 20 percent down payment.
Freddie Mac collects mortgage rates on Monday through Wednesday of each week from lenders across the country. Rates often fluctuate significantly, even within a given day, often tracking long-term Treasury bonds.
The average rate on a 15-year fixed-rate mortgage fell to a record low of 4.27 percent, from 4.29 percent last week, according to Freddie Mac.
Rates on five-year, adjustable-rate mortgages averaged 4.19 percent, up from 4.18 percent a week earlier. Rates on one-year, adjustable-rate mortgages fell to 4.25 percent from 4.35 percent.
The rates do not include add-on fees known as points. One point is equal to 1 percent of the total loan amount.
The nationwide fee for loans in Freddie Mac's survey averaged 0.7 points for 30-year loans. The fee averaged 0.6 points for 15-year, five-year and one-year loans.
Buyers and homeowners who want to refinance are picking up their phones. Mortgage applications rose 2 percent last week from a week earlier, the Mortgage Bankers Association said Wednesday, driven by a more than 4 percent increase in purchase applications and a nearly 2 percent increase in applications to refinance existing loans.